This invention relates to a method of providing communications security, and more particularly, to a method of providing communications security in a multiplexed communications system.
In modern communications systems, multiplexing is commonly used in order to make maximum use of the available band width. Such a technique is particularly useful in communications satellite networks. Shown in FIG. 1 is a brief block diagram of a typical broadcast satellite network in which the signals from a plurality of sources S.sub.1 -S.sub.n are combined by multiplexer 10 into a single communication signal which is then transmitted by a modem 12 to a satellite 14. The satellite retransmits the multiplexed signal to each of the receiving modems 16 which, in turn, provide the received signal to demultiplexers 18. The demultiplexers 18 separate from the multiplexed signal the portion intended for their respective destinations D.sub.1 -D.sub.n. This type of operation is quite well known.
In some applications, it is desirable to encode some or all of the transmitted information in order to prevent access to that information by unauthorized persons. However, the encoding and decoding of multiplexed communications signals has been difficult to accomplish satisfactorily. There are two commonly used techniques for providing communication security in broadcast networks. One is to encipher the messages before multiplexing at the point designated by reference numerals 1(a) in FIG. 1 and to decipher the messages after demultiplexing at the point shown by reference numeral 1(b) in FIG. 1. The disadvantage of this technique is that the information from each source must be completely enciphered prior to the multiplexing operation and, therefore, each source must have its own complete enciphering equipment. A further disadvantage is that the encryption and decryption of each channel is completely independent of all other multiplexed channels. With each channel thus exposed to isolated cryptanalysis attack, it may be necessary to utilize a rather complicated encryption scheme for each channel.
A second commonly used technique is to encipher after the multiplexing operation as shown by reference numeral 2(a) in FIG. 1 and to then decipher prior to demultiplexing as shown by reference numeral 2(b) in FIG. 1. The major disadvantage of the second technique is that the enciphering equipment must have the capability of recognizing the destination of each individual message within the common multiplexed signal and change the enciphering keys dynamically if it is desirable to use different enciphering and deciphering keys for different destinations. If a somewhat sophisticated encryption scheme is desired to maintain communication security the requirement that such a sophisticated apparatus be capable of dynamically changing enciphering keys may result in a rather costly apparatus. Also, this second technique suffers from the same disadvantage as the first technique in that the encryption and decryption of each individual message within the multiplexed signal is independent of the other messages within the signal, thus exposing each message to an isolated cryptanalysis attack.